The present invention relates to a flow control process and apparatus for fluid delivery systems in which fluid is dispensed from a fluid reservoir or container under the control of a pump. More particularly, the invention relates to controlling the rate of delivery of fluid administered to a patient by an intravenous infusion system utilising a standard administration set.
Many applications in the field of intravenous infusion therapy require precise control of the accuracy in administering both the total volume to be infused and the rate at which infusion takes place. Such precise infusions are usually undertaken with the aid of devices commonly referred to as "volumetric infusion pumps" wherein accurate delivery is achieved by means of precisely made sections of tubing or cylinders which contain a known volume of fluid and from which this fluid is discharged at a known rate, thereby accurately dispensing specific volumes of fluid at preset delivery rates.
Whilst satisfactory in operation, the main disadvantage of such infusion pumps is the need for special tubes or cylinders which, once used, must be discarded for reasons of sterility. Compared, therefore, with a conventional gravity infusion apparatus, which employs a cheap standard administration set, the cost per infusion using a volumetric infusion pump is relatively expensive. Furthermore, the disposables inventory at a medical establishment must be increased to include special items required by these volumetric pumps.
Normal gravity infusion, although low in cost owing to the use of mass-produced disposable standard administration sets, is totally unsuitable for precisely controlled infusions because the accuracy of delivery cannot be practically controlled even with the aid of a conventional flow controller. Similarly, the "drip-rate" type of device, over which control is maintained by counting drops, is capable of providing an accurate drop delivery rate, often with standard administration sets, but cannot provide volumetric accuracy owing to the wide variation in drop sizes.
For medical purposes, it is unacceptable to monitor flow-rate by means of a metering device introduced into a fluid stream being intravenously administered because this violates the required sterile barrier. Measurement must therefore be external to this barrier. Equally, measurement by passing fluid into a graduated container is unacceptably cumbersome and, without the addition of expensive equipment, precludes automatic calibration.
One type of pump which does not violate the required sterile barrier is a peristaltic pump. The latter can be arranged to deliver fluid using a standard administration set and may be provided with controls which regulate the speed of the motor and, hence, the pumping rate during operation of the pump. Adjustment of the controls to vary the speed of the motor changes the rate of fluid delivery and the pump may incorporate a visual display to indicate to the operator the rate of fluid flow for a selected position of the controls. The pump may also include a facility for adjusting the display to conform to the actual pumping performance of the tube on which it is operating by measurement of a known volume in a known time. This synchronising of actual performance with displayed performance is known as calibration.
Once calibrated, it would be reasonable to expect the actual delivery rate, over the speed range of the motor, when operating on other administration sets of the same type to comply with that selected by the controls and indicated by the display and, within certain limits, this is normally the case. However, variations in the delivery performance of tubing of the type used on standard administration sets are such that repeatability of calibrated performance on different samples of the same tube and in different environments is insufficient to maintain volumetric accuracy of the delivery for precise infusion applications, as referred to above. Moreover, the performance of tubing used in administration sets supplied from different sources can vary markedly. There are many reasons why the performance of tubing varies when used for pumping applications, but there is no need to expand on these in the present specification. Suffice it to say that unacceptable variations occur and these have hitherto precluded the use of peristaltic pumps for infusions requiring precise volumetric delivery, even when calibrated for the type of tube in actual use.
Variations in tubing performance can largely be eliminated if calibration is carried out directly on the tubing to be used for each infusion since, in this way, both actual and selected fluid delivery can be synchronised with accuracy at the outset of the infusion. However, this approach is tedious and time consuming and is still subject to error because the delivery capacity of the tubing can alter during infusion for various mechanical and environmental reasons.
Having regard to the foregoing, it will be apparent that apparatus in which accurate volumetric delivery can be achieved with standard administration sets would be of considerable advantage in reducing the cost per infusion and alleviating the need for high-cost special disposable items in the inventory of medical establishments.
Several arrangements have been proposed for achieving accurate volumetric delivery with intravenous administration systems, some utilising standard administration sets. Examples of such systems are described in patent specifications Nos. CH-A-600892, GB-A-2054200 and WO-A-82/03554. Basically, all these systems monitor the changing weight of a fluid container or bag as fluid is dispensed and produce weight signals corresponding to the loss in weight. These weight signals are compared with a signal identifying a user selected delivery rate and the resulting signal is used to adjust a flow controller or pump so that the actual delivery rate approaches or corresponds to the selected rate.
Other patent specifications of background interest with regard to the control of fluid delivery rates are GB-A-No. 1117278, U.S. Pat. No. 3,855,458 and GB-A-No. 2069063.
The hitherto known systems which monitor weight loss in order to achieve control of fluid delivery rate do not exercise control over the delivery rate until some interval of time after the start of a dispensing operation when the first weight signal is produced corresponding to the first weight increment loss of the fluid container. Furthermore, they are unlikely to maintain delivery rates continuously constant and avoid fluctuations, as the flow control devices employed are generally actuated on an "open-and-shut" or "on-off" basis. Hence, the known systems suffer from a lack of precision in the control of fluid delivery rate at subsequent stages in a dispensing operation, and they exercise no control of the delivery rate upon start of the operation. This lack of control may not be important for an intravenous feeding system but it is a problem in drug infusion and other administration systems where fluid is administered to a patient at low infusion rates. In these circumstances, for example, an interval of several or more minutes may occur before an acceptable weight signal is produced for comparison with the selected delivery rate signal and the weighing system commences to exercise control of the delivery rate.
In an apparent attempt to alleviate the above problem, WO-A-No. 82/03554 describes a fluid flow control system in which weight loss control circuitry of the type described is combined with drop-rate control circuitry. The combination is said to provide better instantaneous control of the fluid flow. However, such an arrangement must suffer from the same problem, referred to above, as conventional drop rate devices and cannot provide accurate delivery rates owing to the wide variation in drop sizes.